I had a similar problem early on with my landings.
The instructor took control on one landing and told me I was holding the yoke as if it was a cobra about to strike me!
I used to sweat so much that I found it difficult to hold the controls,I bought a pair of thin riding gloves,about £6 a pair which allowed me to keep a more gentle grip and it all began to get better after that.
I also found that if I was distracted ,for example by map reading, my altitiude could drop a couple of hundred feet.
Sounds like you have a good instructor and I can't give any other advice as I'm only just qualified, but it does get easier,honestly!
Lister
